In general, a liquid developer for electrostatic photography (electrophotography) is prepared by dispersing an inorganic or organic pigment or dye such as carbon black, nigrosine, phthalocyanine blue, etc., a natural or synthetic resin such as an alkyd resin, an acrylic resin, rosine, synthetic rubber, etc., in a liquid having a high electric insulating property and a low dielectric constant, such as a petroleum aliphatic hydrocarbon, etc., and further adding a polarity-controlling agent such as a metal soap, lecithin, linseed oil, a higher fatty acid, a vinyl pyrrolidone-containing polymer, etc., to the resulting dispersion.
In such a developer, the resin is dispersed in the form of insoluble latex grains having a grain size of from several nm to several hundred nm. In a conventional liquid developer, however, a soluble dispersion-stabilizing resin added to the liquid developer and the polarity-controlling agent are insufficiently bonded to the insoluble latex grains, thereby the soluble dispersion-stabilizing resin and the polarity-controlling agent are in a state of easily dispersed in the liquid carrier. Accordingly, there is a fault that when the liquid developer is stored for a long period of time or repeatedly used, the dispersion-stabilizing resin is split off from the insoluble latex grains, thereby the latex grains are precipitated, aggregated, and accumulated to make the polarity thereof indistinct. Also, since the latex grains once aggregated or accumulated are reluctant to re-disperse, the latex grains remain everywhere in the developing machine attached thereto, which results in causing stains of images formed and malfunctions of the developing machine, such as clogging of a liquid feed pump, etc.
For overcoming such defects, a means of chemically bonding the soluble dispersion-stabilizing resin and the insoluble latex grains is disclosed in U.S. Pat. No. 3,990,980. However, the liquid developer disclosed therein is still insufficient although the dispersion stability of the grains to the spontaneous precipitation may be improved to some extent. When the above liquid developer is actually used in a developing apparatus, there are same defects that the toner attached to parts of the developing apparatus is solidified in the form of coating and the toner grains thus solidified are reluctant to re-disperse and are insufficient in re-dispersion stability for practical use, which causes the malfunction of the apparatus and staining of duplicated images.
In the method for producing resin grains described in aforesaid U.S. Pat. No. 3,990,980, there is a very severe restriction in the combination of a dispersion stabilizer being used and monomer(s) being insolubilized for producing monodispersed latex grains having a narrow grain size distribution. Mostly, the resin grains produced by the aforesaid method are grains of a broad grain size distribution containing a large amount of coarse grains or poly-dispersed grains having two or more different mean grain sizes. In the aforesaid method, it is difficult to obtain monodispersed resin grains having a narrow grain size distribution and having a desired grain size, and the method often results in forming large grains having a mean grain size of 1 [m or larger or very fine grains having a mean grain size of 0.1 .mu.m or less. Furthermore, there is also a problem that the dispersion stabilizer being used must be prepared by an extremely complicated process requiring a long reaction time.
Furthermore, for overcoming the above problems, a method for improving the dispersibility, redispersibility and storage stability of insoluble dispersion resin grains by forming the resin grains with a copolymer of a monomer imparting insolubility and a monomer having a long chain alkyl moiety or a monomer having two or more kinds of polar components is disclosed in JP-A-60-179751 and JP-A-62-151868 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Also, a method for improving the dispersibility, redispersibility, and storage stability of insoluble dispersion resin grains by polymerizing a monomer imparting insolubility in the presence of a polymer utilizing di-functional monomer or a polymer formed by utilizing a macromolecular reaction to produce the insoluble dispersion resin grains is disclosed in JP-A-60-185962 and JP-A-61-43757.
On the other hand, a method of printing a large number of prints (e.g., 5,000 or more prints) has recently been developed, using an offset printing master plate by electrophotography. In particular, with the improvement of master plates, it has become possible to print 10,000 or more prints of large size in electrophotographic system. In addition, a noticeable progress has been made in shortening the operation time in an electrophotomechanical system and a system of quickening the development-fixing step has been improved.
The dispersion resin grains prepared by the methods disclosed by aforesaid JP-A-60-179751 and JP-A-62-151868 might be good in the mono-dispersibility, redispersibility, and storage stability of the resin grains, but showed unsatisfactory performance with respect to the printing durability for master plates of large size and quickening of the fixing time.
Also, the dispersion resin grains prepared by the methods disclosed in aforesaid JP-A-60 185962 and JP-A-61-43757 were not always satisfactory in the points of the dispersibility and redispersibility of the resin grains when the development speed was increased and also in the point of the printing durability when the fixing time was shortened or a master plate of a large size (e.g., larger than 297.times.420 mm.sup.2) was employed.